Inertia welding is a known process for joining two metallic components. During an inertia welding process, a first component is mounted for rotation coaxially with a flywheel which is driven up to a predetermined rotational speed. Rotary drive is then removed from the flywheel and the rotating first component is brought into contact with a static second component. Pressure is applied to force the two components together. Energy stored in the flywheel continues to cause rotation of the first component and the resulting friction between the components generates heat. The heat generated is sufficient both to soften the interface between the components and to assist the applied pressure in achieving a solid phase weld.
During the inertia welding process material is displaced, or upset, resulting in a shortening of the overall length of the two components. The total amount by which the length of the two components is reduced is termed the “upset length”. Achieving an upset length within tight tolerances is important in many industries, including the aerospace industry, and it is known to control a friction welding process in an effort to achieve a desired upset length.
Existing methods of upset control compare the real time weld upset curve (graph of upset length against time) with the “ideal” target upset curve. As variations from the ideal curve are detected, either the weld forge pressure, or the rotational speed of the flywheel, is modulated accordingly. Such existing methods are necessarily reactive, only capable of reacting to weld data relatively late in the formation of the weld, once upset has already begun to take place. Considerable variation of upset length can still occur, as illustrated for example in FIG. 1, and there is therefore a need for an improved method of controlling upset.